From mboxrd@z Thu Jan 1 00:00:00 1970 From: Torvald Riegel Subject: Re: [RFC][PATCH 0/5] arch: atomic rework Date: Mon, 03 Mar 2014 21:46:19 +0100 Message-ID: <1393879579.28840.11949.camel@triegel.csb> References: <20140224185341.GU8264@linux.vnet.ibm.com> <1393515453.28840.9961.camel@triegel.csb> <20140227190611.GU8264@linux.vnet.ibm.com> <20140227205312.GX8264@linux.vnet.ibm.com> <20140301005047.GA14777@linux.vnet.ibm.com> <1393872908.28840.11660.camel@triegel.csb> <20140303192026.GO11910@linux.vnet.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Return-path: List-Unsubscribe: List-Archive: List-Post: List-Help: Sender: gcc-owner@gcc.gnu.org In-Reply-To: <20140303192026.GO11910@linux.vnet.ibm.com> To: paulmck@linux.vnet.ibm.com Cc: Linus Torvalds , Will Deacon , Peter Zijlstra , Ramana Radhakrishnan , David Howells , "linux-arch@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "akpm@linux-foundation.org" , "mingo@kernel.org" , "gcc@gcc.gnu.org" List-Id: linux-arch.vger.kernel.org On Mon, 2014-03-03 at 11:20 -0800, Paul E. McKenney wrote: > On Mon, Mar 03, 2014 at 07:55:08PM +0100, Torvald Riegel wrote: > > xagsmtp2.20140303190831.9500@uk1vsc.vnet.ibm.com > > X-Xagent-Gateway: uk1vsc.vnet.ibm.com (XAGSMTP2 at UK1VSC) > > > > On Fri, 2014-02-28 at 16:50 -0800, Paul E. McKenney wrote: > > > +o Do not use the results from the boolean "&&" and "||" when > > > + dereferencing. For example, the following (rather improbable) > > > + code is buggy: > > > + > > > + int a[2]; > > > + int index; > > > + int force_zero_index = 1; > > > + > > > + ... > > > + > > > + r1 = rcu_dereference(i1) > > > + r2 = a[r1 && force_zero_index]; /* BUGGY!!! */ > > > + > > > + The reason this is buggy is that "&&" and "||" are often compiled > > > + using branches. While weak-memory machines such as ARM or PowerPC > > > + do order stores after such branches, they can speculate loads, > > > + which can result in misordering bugs. > > > + > > > +o Do not use the results from relational operators ("==", "!=", > > > + ">", ">=", "<", or "<=") when dereferencing. For example, > > > + the following (quite strange) code is buggy: > > > + > > > + int a[2]; > > > + int index; > > > + int flip_index = 0; > > > + > > > + ... > > > + > > > + r1 = rcu_dereference(i1) > > > + r2 = a[r1 != flip_index]; /* BUGGY!!! */ > > > + > > > + As before, the reason this is buggy is that relational operators > > > + are often compiled using branches. And as before, although > > > + weak-memory machines such as ARM or PowerPC do order stores > > > + after such branches, but can speculate loads, which can again > > > + result in misordering bugs. > > > > Those two would be allowed by the wording I have recently proposed, > > AFAICS. r1 != flip_index would result in two possible values (unless > > there are further constraints due to the type of r1 and the values that > > flip_index can have). > > And I am OK with the value_dep_preserving type providing more/better > guarantees than we get by default from current compilers. > > One question, though. Suppose that the code did not want a value > dependency to be tracked through a comparison operator. What does > the developer do in that case? (The reason I ask is that I have > not yet found a use case in the Linux kernel that expects a value > dependency to be tracked through a comparison.) Hmm. I suppose use an explicit cast to non-vdp before or after the comparison? From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx1.redhat.com ([209.132.183.28]:4666 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753787AbaCCUq4 (ORCPT ); Mon, 3 Mar 2014 15:46:56 -0500 Subject: Re: [RFC][PATCH 0/5] arch: atomic rework From: Torvald Riegel In-Reply-To: <20140303192026.GO11910@linux.vnet.ibm.com> References: <20140224185341.GU8264@linux.vnet.ibm.com> <1393515453.28840.9961.camel@triegel.csb> <20140227190611.GU8264@linux.vnet.ibm.com> <20140227205312.GX8264@linux.vnet.ibm.com> <20140301005047.GA14777@linux.vnet.ibm.com> <1393872908.28840.11660.camel@triegel.csb> <20140303192026.GO11910@linux.vnet.ibm.com> Content-Type: text/plain; charset="UTF-8" Date: Mon, 03 Mar 2014 21:46:19 +0100 Message-ID: <1393879579.28840.11949.camel@triegel.csb> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-arch-owner@vger.kernel.org List-ID: To: paulmck@linux.vnet.ibm.com Cc: Linus Torvalds , Will Deacon , Peter Zijlstra , Ramana Radhakrishnan , David Howells , "linux-arch@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "akpm@linux-foundation.org" , "mingo@kernel.org" , "gcc@gcc.gnu.org" Message-ID: <20140303204619.bIQa_PMUW7UbO-3LZxvJPmnsKeaBmyd-RFwy1IZZtQU@z> On Mon, 2014-03-03 at 11:20 -0800, Paul E. McKenney wrote: > On Mon, Mar 03, 2014 at 07:55:08PM +0100, Torvald Riegel wrote: > > xagsmtp2.20140303190831.9500@uk1vsc.vnet.ibm.com > > X-Xagent-Gateway: uk1vsc.vnet.ibm.com (XAGSMTP2 at UK1VSC) > > > > On Fri, 2014-02-28 at 16:50 -0800, Paul E. McKenney wrote: > > > +o Do not use the results from the boolean "&&" and "||" when > > > + dereferencing. For example, the following (rather improbable) > > > + code is buggy: > > > + > > > + int a[2]; > > > + int index; > > > + int force_zero_index = 1; > > > + > > > + ... > > > + > > > + r1 = rcu_dereference(i1) > > > + r2 = a[r1 && force_zero_index]; /* BUGGY!!! */ > > > + > > > + The reason this is buggy is that "&&" and "||" are often compiled > > > + using branches. While weak-memory machines such as ARM or PowerPC > > > + do order stores after such branches, they can speculate loads, > > > + which can result in misordering bugs. > > > + > > > +o Do not use the results from relational operators ("==", "!=", > > > + ">", ">=", "<", or "<=") when dereferencing. For example, > > > + the following (quite strange) code is buggy: > > > + > > > + int a[2]; > > > + int index; > > > + int flip_index = 0; > > > + > > > + ... > > > + > > > + r1 = rcu_dereference(i1) > > > + r2 = a[r1 != flip_index]; /* BUGGY!!! */ > > > + > > > + As before, the reason this is buggy is that relational operators > > > + are often compiled using branches. And as before, although > > > + weak-memory machines such as ARM or PowerPC do order stores > > > + after such branches, but can speculate loads, which can again > > > + result in misordering bugs. > > > > Those two would be allowed by the wording I have recently proposed, > > AFAICS. r1 != flip_index would result in two possible values (unless > > there are further constraints due to the type of r1 and the values that > > flip_index can have). > > And I am OK with the value_dep_preserving type providing more/better > guarantees than we get by default from current compilers. > > One question, though. Suppose that the code did not want a value > dependency to be tracked through a comparison operator. What does > the developer do in that case? (The reason I ask is that I have > not yet found a use case in the Linux kernel that expects a value > dependency to be tracked through a comparison.) Hmm. I suppose use an explicit cast to non-vdp before or after the comparison?